1. Field of the Invention
The present invention relates to an X-ray diffraction measurement method and an X-ray diffraction apparatus both for performing X-ray diffraction measurement by use of a fixed divergence slit.
2. Description of the Related Art
Generally, in an X-ray diffraction measurement, X-ray emitted from an X-ray source is applied to a sample at an incident angle “θ”, and diffracted X-ray emitted from the sample at a diffraction angle “2θ” is detected by an X-ray detector, and intensity I of the diffracted X-ray is calculated based on an output signal of the X-ray detector. Diffraction angle “2θ” is always an angle double the incident angle “θ”. Given that diffracted X-ray intensity I is a function of the X-ray incident angle “θ”, the diffracted X-ray intensity can be expressed by I(θ). On the other hand, given that diffracted X-ray intensity I is a function of diffraction angle “2θ”, the diffracted X-ray intensity can be expressed by I(2θ). In the present specification, the expression of I(θ) is frequently used for convenience' sake.
In the above X-ray diffraction measurement, X-ray emitted from the X-ray source is irradiated on the sample with its size in the sample width direction restricted by a divergence slit (DS). A method in which X-ray diffraction measurement is performed with the slit width (that is the divergence angle) of the divergence slit being fixed has widely been known. For example, section 7-3 “X-ray optics” in page 178 (7-th chapter “Diffractometer and Spectrometer Measurements”) of “Elements of X-ray diffraction, New Edition” (written by B. D. Cullity and translated by Gentaro Matsumura, issued from KK AGNE, Mar. 25, 1989, 7-th edition) discloses that the spread angle (i.e., divergence angle) of a commonly used divergence slit is “1°”, that is, divergence angle is set to a fixed value.
Further, Japanese Unexamined Patent Application Publication No. S50-63982, in column of Prior Art and FIG. 1 (corresponding description is shown in page 2), discloses that the divergence angle of the divergence slit is set to a fixed value. Hereinafter, in the present specification, a divergence slit whose divergence angle is set to a fixed value is referred to as “fixed divergence slit” and a divergence slit whose divergence angle is variable is referred to as “variable divergence slit”.
Conventionally, in X-ray diffraction measurement using the fixed divergence slit, a sample S is disposed at a standard sample area, which is defined by standard sample width Wr×standard sample height Hr, as shown in FIGS. 6A and 6B. In a typical X-ray diffraction apparatus, setting is made such that Wr=Hr=20 mm. FIG. 6B is a cross-sectional view taken along Z6-Z6 line of FIG. 6A. As shown in FIGS. 6A and 6B, X-ray emitted from an X-ray source F is irradiated on the sample S with the divergence thereof restricted by a divergence slit 101. When diffracted X-ray is generated from the sample S, the diffracted X-ray is detected by an X-ray detector 102.
X-ray applied to the sample S is shown rectangularly by a chain line surrounding the sample S in FIG. 6A. The width W0 of X-ray applied to the sample S is determined by divergence angle “β” of the divergence slit 101 and X-ray incident angle “θ”. The width W0 is referred to as “X-ray irradiation width” hereinafter. FIG. 6B shows a case where X-ray incident angle θ is a low angle. In this case, X-ray irradiation width W0 is larger than standard sample width Wr. FIG. 7C is a case where X-ray incident angle θ is the boundary angle between a low angle and high angle. In this case, X-ray irradiation width W0 is equal to sample width. The sample width in FIG. 7C is the standard sample width Wr. FIG. 7D shows a case where X-ray incident angle θ is a high angle. In this case, X-ray irradiation width W0 is smaller than standard sample width Wr.
In FIG. 6A, the height of X-ray applied to the sample S, that is the length of the X-ray in a direction perpendicular to the X-ray irradiation width W0, is illustrated to be larger than the standard sample height Hr for clearly showing them. Actually, the height of X-ray applied to the sample S is substantially same as the standard sample height Hr. Such a condition of the height of X-ray applied to the sample S against the standard sample height Hr also applies to FIGS. 8A, 8B, and 9A, respectively.
In the present specification, the sample disposed in the aforesaid standard sample area is hereinafter referred to as “standard-arrangement sample”. When X-ray diffraction measurement is performed using the fixed divergence slit for the standard-arrangement sample, X-ray incident angle θ is moved from a low angle region shown in FIG. 6B to a high angle region shown in FIG. 7D while the surface of the sample S is scanned with X-ray, and the diffracted X-ray from the sample S is detected by the X-ray detector 102 during the scanning operation.
There may a case where a sufficient amount of sample cannot be prepared for actual X-ray diffraction measurement. In this case, it is impossible to fill up the entire standard sample area (Wr×Hr) shown in FIG. 6A with the sample S. In order to cope with such a case in which the amount of the sample is insufficient, two arrangement methods of the sample shown in FIGS. 8A and 8B are available.
The first method shown in FIG. 8A is a method in which the sample width is made equal to standard sample width Wr and the sample height Hs is made smaller than standard sample height Hr. In the present specification, this arrangement is referred to as “laterally-elongated arrangement”. On the other hand, the second method shown in FIG. 8B is a method in which the sample width Ws is made smaller than standard sample width Wr and the sample height is made equal to standard sample height Hr. In the present specification, this arrangement is referred to as “longitudinally-elongated arrangement”.
In the case of the laterally-elongated arrangement shown in FIG. 8A, a relationship between X-ray irradiation width W0 and sample width Wr in the course of change in X-ray incident angle θ (see FIG. 6B) is the same as that in the case of the standard-arrangement sample which is shown in FIGS. 6B, 7C, and 7D. That is, in this case, the X-ray irradiation width falls equal to or smaller than the sample width in a wide range of the diffraction angle 2θ. The amount of X-ray applied to the sample is constant in the region within which the X-ray irradiation width falls equal to or smaller than the sample width, and relative X-ray intensity (i.e., intensity ratio) of a peak existing in this range is measured correctly. In the diffraction angle region within which the irradiation width exceeds the sample width, the relative intensity decreases since the amount of effective X-ray is reduced.
The relative X-ray intensity of a peak means a ratio of observed X-ray intensity relative to “true” X-ray intensity. The “true” X-ray intensity means X-ray intensity observed in the diffraction angle region within which the X-ray irradiation width falls equal to or smaller than the sample width. That is, when the X-ray irradiation width is within the sample width, an X-ray intensity having the same strength as that of the true X-ray intensity is observed.
In the case of the longitudinally-elongated arrangement, as can be understood from FIGS. 9B, 10C and 10D, X-ray irradiation width W0 runs off sample width Ws even when X-ray incident angle θ is a relatively high angle. When X-ray irradiation width W0 runs off sample width Ws, X-ray is applied to unnecessary portion outside the sample width and, accordingly, the amount of X-ray applied to the sample width is reduced, with the result that the relative X-ray intensity of a peak of the diffracted X-ray is reduced to prevent correct determination of a peak of the diffracted X-ray.
Under such circumstances, when X-ray diffraction measurement is performed for a sample of insufficient amount, those skilled in the art in the field of X-ray measurement generally select the laterally-elongated arrangement in which a correct and constant relative X-ray intensity (namely intensity ratio) in a wide range of the diffraction angle 2θ can be maintained. However, in the case where the laterally-elongated arrangement is adopted, the X-ray irradiation height always runs off sample height Hs in FIG. 8A. This reduces the amount of sample that can contribute to diffraction, thus posing a problem in that a sufficient diffracted X-ray intensity cannot be obtained especially on the high angle side of the X-ray incident angle θ. In order to obtain a sufficient diffracted X-ray intensity in this situation, measurement needs to be performed for a very extended period of time. This is unrealistic.